1. Field of the Invention
The invention is directed toward instrumentation for biochemical assays, more particularly assays using fluorescence detection with two or more labels in the experiment; or assays requiring a ratiometric measurement of fluorescence intensity at two spectral bands.
2. Description of the Related Art
Fluorescence assays that use multiple probes typically require measuring the fluorescence emission levels in two spectral bands, corresponding to the probes involved. Similarly, one may look in two bands to detect shifts in the spectrum of a single probe, if its emission spectrum is sensitive to environment. Examples of such probes include Indo, for pH sensing; or Acridine Orange to sense binding to RNA. In these cases, one wishes to use spectral ranges whose flux levels will be most sensitive to the spectral shift.
In such measurements, one often seeks to compare the flux in levels between the bands with high precision. In an assay looking for spectral shifts in a single probe, the ability to take precise ratios sets the detection limit for how small of a spectral shift can be detected. In dual-probe experiments, measuring the ratio precisely is valuable as well.
Existing fluorescence readers offer one the means to choose what spectral band will be measured, so it is possible to perform a dual band measurement simply by reading the sample twice in series. However, this is undesirable for several reasons. First, it takes twice as long as a single-band measurement, which can result in sample aging. Second, fluorescent probes exhibit photobleaching so the second reading is diminished in intensity, which distorts the measured ratio of fluxes. Finally, lamp flicker and drift are a significant source of noise in fluorescent measurements, and will degrade the measurement of flux ratios between bands.
Some instruments incorporate two or more detectors viewing the same sample simultaneously. These divide the beam according to wavelength band using partial-mirrors or dicroic elements to split of a sample portion to each detector. This approach is superior in the sense that it affords a higher throughput by reading both bands at once; and since the bands are measured under identical illumination conditions, a ratio of the two is not affected by lamp flicker or drift. However, such instruments bear a cost burden due to the need for two detectors and readout electronics, plus beamsplitters. This cost burden becomes even more significant if one wishes to make an imaging system of this type, using two pixelated detectors such as CCD detectors. In addition to the higher cost of the detectors and readout electronics, the two detectors need to be carefully registered in order that one can relate the image elements seen at a particular pixel on a first detector with a given pixel at the other detector. This registration must be handled carefully and while the approach is simple in theory, building such an instrument in practice tends to be demanding.
There is another problem with existing multiband fluorescence instruments. The use of dielectric filters at non-normal incidence to sample the beam, or indeed anywhere in the instrument, can lead to systematic spectral and polarization errors. In general, the off-axis reflection and transmission properties of dielectric film are different for the S- and P-state of polarization. Simply put, one observes a different spectral band in the S-polarization than one does in the P-polarization. In some cases, the difference is not significant because the spectral width of the overall measurement is determined by some other element in the system, such as a normal-incidence filter elsewhere in the system.
In practice, problems are most likely present when one wishes to observe bands that are spectrally adjacent, or where one band is spectrally close to the excitation wavelength. Yet dual-probe assays typically use one probe with a long Stokes shift, and one with a short Stokes shift (i.e. spectrally close to the excitation wavelength). Such samples are not measured accurately by an instrument that uses dielectric filters to split off the beam to multiple detectors. Thus the deficiencies of the prior art instruments are most likely to be germane, precisely when utilizing their dual-band readout capability to read dual-probe assays.
Such a dual-detector approach also would be less than optimal for measuring fluorescence polarization, due to the polarization sensitivity of the instrument. All existing fluorescence polarization instruments operate on a single band at the same time, so they suffer the throughput and noise penalties discussed above in connection with all single-band instruments.
An instrument described in U.S. Pat. No. 6,160,618 issued to Garner uses an imaging spectrometer to obtain a complete spectrum of the fluorescent emission by means of a dispersive element such as a grating or prism. There are several limitations to this instrument. In addition to the complexity of this approach, the instrument can only collect light from a small region corresponding to the image of the spectrometer slit projected on the sample. Thus its sensitivity is relatively low compared to instruments that can illuminate, and collect light from, extended regions such as sample spots or microtitre plate wells. Further, the efficiency of gratings and prisms depends upon polarization state, which renders a dispersive instrument inherently ill-suited for fluorescence polarization assays
Thus there is no instrument for fluorescence assay measurement which at once provides for simultaneous readout of two bands for high throughput and low-noise assessment of band ratios; with the economy of a single detector; or with high accuracy when used with short Stokes-shift probes or for fluorescence polarization measurements.
It is an object of the present invention to provide for an ultra-high-throughput measurements at two bands without the need for multiple detectors that add cost or tilted dielectric filters that degrade accuracy.
It is a further object of the present invention to enable making dual-band ratio measurements with high precision suitable for use with Fluorescence Resonance Energy Transfer (FRET) potentiometric assays or environmentally-sensitive probes.
It is a further object of the present invention to enable performing dual label fluorescence polarization assays to detect single nucleotide polymorphism (SNP).
It is a further object of the present invention 1 to provide for fluorescence polarization measurements with high accuracy, for either a single probe or for two probes at once.
It is yet another object of the present invention to provide for reading more than one sample region at a time, to further increase the throughput.
It is yet another object of the present invention to achieve this in a compact, economical design which has no moving parts.
These and other objects are achieved by the present invention, which provides a system and method for separating fluorescent light emitted from a spot on a sample into multiple spots according to wavelength and, in some embodiments, according to polarization state when emitted by the sample as well. The resultant spots are directed to a multi-pixel detector where their flux is measured.
In one illustrative system, the sample is illuminated with a laser spot, and the fluorescence emitted from the spot is first separated into two spots according to polarization state using a double-refractive element. Each of these passes through a birefringent network which changes the state of polarization to its complement, or not, depending on its wavelength. A second double-refractive element further splits each of the two spots by polarization, which now corresponds to wavelength in a predetermined way; to yield four spots, separated according to wavelength and polarization state at the sample.
This arrangement can be used for dual band fluorescence assays of all kinds. It is ideal for dual-probe fluorescence polarization assays, since it simultaneously captures all ratios of intensity and of wavelength, eliminating lamp drift between measurements as a source of error. Blocking of the excitation source is achieved by a conventional long-pass filter, a holographic notch filter, or other blocking element, as is known in the art of instrument design. It is further possible to use bandpass or multiband filters to further define the spectral bands, which are primarily set by the birefringent network.
In another illustrative system, the second double-refraction element is replaced by a linear polarizer, and two spots corresponding to different wavelengths are presented to different regions of the detector.
It is possible to put a polarization rotator before the first double-refractive element, and thus change which spot at the detector corresponds to what polarization at the sample. It is also possible to put a polarization rotator between the first and second double-refractive elements, and thus to change which spot at the detector corresponds to what wavelength. This can be useful in instrumental calibrations and the like. The polarization rotators can be liquid crystal cells, so interchanging spot locations in this way is rapid and does not require any moving parts.
It is possible to put a pixelated polarization rotator in the path of just one of the two spots at the point between the first and second double-refractive element, and thus to interchange the location of only two of the spots at the detector. In this way one can arrange the spots so that e.g. all spots with a given wavelength are adjacent, or all spots of a given polarization are adjacent. This enables high-speed readout when only polarization information, or only wavelength information, is of interest.
Since all spots are delivered to a single image plane, they may be read out with a single pixelated detector, such as a CCD, linear photodiode array, or pixelated Photomultiplier tube (PMT). In versions that use a double-refractive element rather than a linear polarizer as the second element, all the fluorescent flux is utilized for the measurement and high optical efficiency can be attained.
It is possible to make all these measurements for plural points on the sample, by choice of suitable illumination optics that excite more than one sample region at once. This multiplies the measurement throughput by the number of illumination spots, for high throughput and ultra-high throughput applications. Using diffractive optical elements it is possible to produce 16 spots or more at the sample, from a single laser source.
This invention is valuable when precise dual-label fluorescence assays and fluorescence polarization assays are needed, as in pharmacogenomics. It is also valuable in taking emission intensity ratios, as in Fluorescence Resonance Energy Transfer (FRET) and membrane potential experiments.
The ability to obtain dual-band spectral data without a significant burden in cost, throughput, or optical efficiency, makes it practical to obtain a second band for purposes other than imaging two probes. For example, the instrument may be set to image a primary band corresponding to the probe of interest, and a secondary band corresponding to background fluorescence. Having an in-situ measure of background fluorescence at the secondary band, together with some knowledge of the spectral distribution of background fluorescence, enables improved background correction at the primary (probe) band. Thus even single-probe measurements can be improved by the present invention.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.